In an unconstrained environment, the precise and repeatable determination of the vehicle's position is a non-trivial problem. The typical solution to determining global vehicle position is to use GPS. Without differential corrections GPS has position errors in the tens of meters. Where WAAS differential corrections are available, GPS position error can be reduced to a few meters. To get down to sub-meter accuracy, the use of differential and phase-differential GPS corrections are necessary. One problem with differential GPS is its general availability. It is necessary to have a differential base station located in the area of operation, which can be impractical or impossible at times.
Other factors that degrade the suitability of GPS for precision vehicle-position sensing are RF multi-path and occlusion issues. GPS relies on RF signals from satellites orbiting the earth. The position of the GPS receiver is determined by very precise measurements of the distance between the visible satellites and the receiver. In obstructed areas such as urban environments and heavily wooded areas the signals from the satellites can be blocked from sight. An accurate position cannot be computed unless enough satellites can be seen by the receiver.
The accuracy of the position solution can also be compromised if the signals from the satellites reach the receiver by an indirect route, for example by bouncing off of buildings. In this case the distances to the satellites will be incorrect, and the resulting position calculated from these measurements will be in error. Another complicating factor is vehicle speed. Typically GPS units provide better results when they are stationary. However, even with its limitations, GPS is the best method for determining global vehicle position.